MINOR: config: add a global directive to set default SSL curves

This commit adds a new keyword to the global section to set default
curves for ssl binds:
  - ssl-default-bind-curves
diff --git a/doc/configuration.txt b/doc/configuration.txt
index a6ff8df..f3e6aa1 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -622,6 +622,7 @@
    - stats
    - ssl-default-bind-ciphers
    - ssl-default-bind-ciphersuites
+   - ssl-default-bind-curves
    - ssl-default-bind-options
    - ssl-default-server-ciphers
    - ssl-default-server-ciphersuites
@@ -1271,6 +1272,13 @@
   "ssl-default-bind-ciphers" keyword. Please check the "bind" keyword for more
   information.
 
+ssl-default-bind-curves <curves>
+  This setting is only available when support for OpenSSL was built in. It sets
+  the default string describing the list of elliptic curves algorithms ("curve
+  suite") that are negotiated during the SSL/TLS handshake with ECDHE. The format
+  of the string is a colon-delimited list of curve name.
+  Please check the "bind" keyword for more information.
+
 ssl-default-bind-options [<option>]...
   This setting is only available when support for OpenSSL was built in. It sets
   default ssl-options to force on all "bind" lines. Please check the "bind"